It is known to build single screw compressors using slides in the casing to change the capacity of the compressor.
Such arrangements are shown on many patents such as U.S. Pat. Nos. 4,261,691, 4,579,513 or 5,087,182.
One serious problem encountered by these compressors happens when the compressor is used in a refrigeration or air-conditioning application and is started partly or totally filled with liquid refrigerant. Such phenomenon is well known in the industry and can be created by many circumstances: a leaking injection valve, a compressor being cooler than the evaporator, etc.
If the compressor is started with enough liquid in it, the general result is a degradation of the performances of the compressor, degradation which is the consequence of considerable pressure generated into the grooves of the screw which have to compress a non-compressible fluid, the liquid refrigerant, instead of the compressible refrigerant gas; such compression of liquid is also known as "liquid lock" as sometimes the motor cannot turn the compressor and stalls.
A solution generally used in screw compressors is to start the compressor with the slides in a position where the compressor capacity is minimum, generally around 15 to 20% of the full load.
In such case indeed, as soon as the groove stops to be in communication with suction, it starts to register with the discharge port; so there is no compression happening in a groove and it can pump liquid without encountering a liquid lock.
This solution has nevertheless many drawbacks; first of all, this obliges generally to provide a control for the slide with at least three positions: starting position with 15% capacity, a part load position with around 40 or 50% capacity and a full load position (a compressor with only a 15% capacity position and full load would have too much change between these two levels); as the slide is most often controlled by a piston, this means that the control must provide three positions which is much more complex than a piston having simply two positions achieved by applying or not a pressure to the piston. Another solution is to provide the piston with an infinite number of positions by a continuous control which is even more complex.
A second drawback is that, at 15% capacity, the compressor builds up pressure in the condenser very slowly; as the compressor pressure is generally used to move the liquid injected into the compressor to cool it, whether oil or liquid refrigerant, there is a risk of lack of liquid injection resulting in a compressor seizure.
In oil injection free compressors (OIF compressors) where there is no oil injected in the compressor but liquid refrigerant from the condenser, it is practically mandatory to start the compressor at around 50% capacity so as to build up pressure fast and start liquid injection within approximately 10 seconds.
In the new generations of single screw compressors where the two slides operate independently, one being fully unloaded at start-up, this implies that the other slide has to start in the full load position; if there is liquid, as compression occurs on the full length of the groove, liquid lock occurs with all its consequences.